(a) Die Cushion Device of Hydraulic (Servo) Type
PTL 1 discloses a die cushion device of a hydraulic servo type that performs throttle control with a proportional valve.
In this die cushion device, a proportional valve is placed on the lower chamber side of a hydraulic cylinder supporting a cushion pad, and a desired die cushion pressure is generated by controlling the proportional valve to have an appropriate aperture.
This die cushion device has the advantages that: the die cushion pressure can be controlled with the use of the proportional valve or the like; a pressure change can be caused as needed; and the diameter of the hydraulic cylinder can be smaller, which implements a pit-less device or smaller devices, since the die cushion device can be used at a relatively high pressure.
On the other hand, this die cushion device has the disadvantage that all the energy used for the die cushioning function is converted into heat, since a pressure is generated by reducing the oil flow. Also, it is necessary to prepare a cooling function (a cooling system) that is compatible with the capability of the device, though using such a cooling system is considered a waste, in view of the environmental friendliness. This applies to devices of all hydraulic types. When the slide velocity is low during a die cushioning operation, compression of the oil in the hydraulic cylinder becomes slower, and the booster responsiveness is more likely to become lower (the boosting time tends to become longer).
(b) Die Cushion Device of Electric (Servo) Type
PTL 2 discloses a die cushion device of an electric servo type.
In this die cushion device, the discharge outlet of a hydraulic pump/motor is connected directly to the lower chamber of a hydraulic cylinder supporting a cushion pad. The torque of an electric motor connected to the rotating shaft of the hydraulic pump/motor is controlled, and the pressure in the lower chamber of the hydraulic cylinder (the die cushion pressure) can be freely controlled.
This die cushion device has an advantage that the energy required for the die cushioning function to which the cushion pad is subjected when the press machine executes the die cushioning function is regenerated as an electric energy via the hydraulic cylinder, the hydraulic pump/motor, and an electric motor, and accordingly, a higher energy efficiency is achieved. In addition, even if the slide velocity is low, the die cushion pressure can be suitably controlled, and a higher pressure controllability than that of a die cushion device of a hydraulic (servo) type can be achieved.
On the other hand, this die cushion device has a disadvantage that a large-capacity electric motor is required to generate the necessary power for the die cushioning function at the same time when the die cushioning function is performed. If the electric motor has a larger capacity, the device becomes larger in size, and the power receiving facilities also require a large capacity. As a result, the system inevitably becomes complicated, and the price becomes higher. Therefore, the die cushion device of the electric servo type is an inefficient device in terms of capital investment, though such a die cushion device has a high energy efficiency.
In addition, in order to release the oil pushed away (displaced) from the hydraulic cylinder to the low-pressure side via the hydraulic pump/motor (and the electric motor) at the time of impact, the angular velocity of the motor (the inertia moment) needs to be rapidly accelerated by the displaced oil, and a surge pressure is easily generated as a reaction to the acceleration.